The ability to systematically modify proteins through site directed mutagenesis has transformed protein science, enabling structure-function studies to be performed in a routine manner. However, despite the utility of these methods, there is a growing demand for proteins that are modified in a manner incompatible with normal cellular ribosomal expression. Chemical protein synthesis is attractive for these studies since there are very few limitations on the structural perturbations that can be introduced. As a result, proteins have been synthesized incorporating unnatural amino acid side chains, backbone elements, site-specific post- translational modifications and biophysical probes and have provided numerous unique insights into the molecular basis of protein function. We have addressed the challenge of synthesizing these large organic molecules through the development of chemical ligation strategies that facilitate the conjugation of unprotected peptides derived from solid phase peptide synthesis. However, despite the demonstrated utility of these methods, synthetic protein chemistry remains a challenging endeavor that is practiced by a relatively few number of laboratories. In this proposal, we describe a comprehensive strategy to refine the existing tools of synthetic protein chemistry and introduce novel synthetic approaches with the aim of developing efficient and high yielding syntheses of proteins. Specifically, we propose the development of novel selenium containing amino acids and N-terminal auxiliary groups to facilitate the chemoselective linking of unprotected polypeptides in aqueous solution. To complement these methods, we further develop an approach for the electrophilic thioester peptides using Fmoc based solid phase peptide synthesis approaches. Together, these methodological advances promise to significantly extend the use of chemistry to address questions in protein science. We will test the utility of these methods in the synthesis of several proteins including dihydrofolate reductase, single chain antibodies and alpha synuclein. Overall, we aim to develop and refine innovative synthetic protein chemistry methods with the goal of making protein synthesis a robust and general methodology for the introduction of non-coded probes into biologically interesting and synthetically challenging protein systems.